German patent application No. 39 25 828 discloses a circuit arrangement of this type. An anti-lock control system with individual control of the braking pressure on both wheels of an axle is described. The braking pressure difference on the two wheels of an axle is restricted to limit the yawing torque on split road surfaces, and the allowable braking pressure difference is determined as a function of the difference in friction coefficients and the magnitude of the lower coefficient of friction. In the event of exceeding of the allowable braking pressure difference, in consideration of the motional condition of the LM-wheel, that is the wheel on the lower coefficient side, the braking pressure on the HM-wheel, i.e. the other wheel of the same axle, is reduced. To determine the allowable pressure difference in this known anti-lock control system, the pressure introduced by the driver is measured on the right and the left wheel, and the nominal pressure is compared to the actual pressure in each case. The coefficient of friction is assessed on the basis of the braking pressure.
Further, German patent application No. 41 14 734 discloses a circuit arrangement for anti-lock control with an individual braking pressure control and yawing torque limitation which is based on the fact that a value, representative of the pressure difference on the two wheels of an axle, is constantly determined from the pressure reduction signals and, under .mu.-split conditions, the medium pressure increase gradient on the HM-wheel is varied as a function of the pressure difference and the vehicle deceleration and that the braking pressure on the HM-wheel is reduced by a value, which is responsive to the vehicle deceleration and the pressure difference, at the time of the occurrence of the so-called yawing torque peak.
The above mentioned, known provisions do not permit preventing the generation of yawing torques of a dangerous amount in defined, especially critical situations. For example, different wheel lock pressure levels in the individual wheel brakes occur on road surfaces having varying coefficients of friction, so-called .mu.-patch road surfaces, due to a changing dynamic axle load distribution and the related variation of the wheel vertical forces. When these variations occur on the right and the left road surface side, they cause yawing torques which may jeopardize the driving stability. Especially in vehicles having a short wheelbase and front-wheel drive, the rear axle alone cannot ensure that driving stability is maintained.
In principle, the previous solutions involve slowing down the occurrence of yawing torques during braking on .mu.-split road surfaces by a decelerated braking pressure rise on the front HM wheel to give the driver time for a reaction, i.e., for countersteering. However, when the coefficient of friction changes, in particular from a high value to a low value, the described effects will be encountered.